2019
Publication Year
2021-02-26T10:17:05Z
Acceptance in OA@INAF
þÿFirst [N II]122 ¼m Line Detection in a QSO-SMG Pair BRI 1202-0725 at z = 4.69
Title
Lee M. M.; Nagao T.; De Breuck C.; Carniani S.; CRESCI, GIOVANNI; et al.
Authors
10.3847/2041-8213/ab412e
DOI
http://hdl.handle.net/20.500.12386/30636
Handle
THE ASTROPHYSICAL JOURNAL LETTERS
First
[N
II
]122 μm Line Detection in a QSO-SMG Pair BRI 1202−0725 at z=4.69
Minju M. Lee1,2,3 , Tohru Nagao4 , Carlos De Breuck5 , Stefano Carniani6 , Giovanni Cresci7 , Bunyo Hatsukade8 , Ryohei Kawabe9,10,11 , Kotaro Kohno8,12 , Roberto Maiolino13,14, Filippo Mannucci7 , Alessandro Marconi7,15 , Kouichiro Nakanishi3,10 , Toshiki Saito16 , Yoichi Tamura2 , Paulina Troncoso17, Hideki Umehata8,18 , and Min Yun19
1
Max-Planck-Institut für Extraterrestrische Physik(MPE), Giessenbachstr., D-85748 Garching, Germnay;minju@mpe.mpg.de
2
Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan 3
National Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
4Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan 5
European Southern Observatory, Karl Schwarzschild Straße 2, D-85748 Garching, Germany 6
Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy 7
INAF—Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-20125 Firenze, Italy 8
Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan 9
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 10
SOKENDAI(The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 11
Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 12
Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan 13
Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK 14
Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
15Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via G. Sansone 1, I-50019 Sesto F.no, Firenze, Italy 16
Max-Planck Institute for Astronomy, Königstuhl, 17 D-69117 Heidelberg, Germany 17
Universidad Autónoma de Chile, Chile, Av. Pedro de Valdivia 425, Providencia, Santiago de Chile, Chile 18
RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 19
Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
Received 2019 April 17; revised 2019 September 3; accepted 2019 September 3; published 2019 September 24
Abstract
We report thefirst detection obtained with the Atacama Large Millimeter/submillimeter Array of the [NII] 122 μm
line emission from a galaxy group BRI 1202−0725 at z=4.69 consisting of a quasi-stellar object (QSO) and a submillimeter-bright galaxy(SMG). Combining this with a detection of [NII] 205 μm line in both galaxies, we
constrain the electron densities of the ionized gas based on the line ratio of[NII] 122/205. The derived electron
densities are26-+11 12 and
-+
134 39
50 cm−3for the SMG and the QSO, respectively. The electron density of the SMG is similar to that of the Galactic Plane and to the average of the local spirals. However, higher electron densities(by up to a factor of three) could be possible for systematic uncertainties of the line flux estimates. The electron density of the QSO is comparable to high-z star-forming galaxies at z=1.5–2.3, obtained using rest-frame optical lines and with the lower limits suggested from stacking analysis on lensed starbursts at z=1–3.6 using the same tracer of [NII]. Our results suggest a large scatter of electron densities in global scale at fixed star formation rates for
extreme starbursts. The success of the[NII] 122 μm and 205 μm detections at z=4.69 demonstrates the power of
future systematic surveys of extreme starbursts at z>4 for probing the interstellar medium conditions and the effects on surrounding environments.
Key words: galaxies: evolution– galaxies: high-redshift – galaxies: ISM – galaxies: starburst – quasars: general – submillimeter: galaxies
1. Introduction
Understanding the physical conditions of star formation is critical in constraining theoretical models of galaxy evolution. Galaxies form stars at a higher rate in the early universe at afixed mass. A subsequent question is how the the interstellar medium (ISM) properties are correspondingly changed to understand the cosmic evolution. Observations of z>1 star-forming galaxies suggest that the ISM state and/or the hardness of the extreme ultraviolet (EUV) radiation field were more extreme in the past than in the present day. For example, rest-frame optical line observations revealed that electron densities of high-redshift star-forming galaxies range between 100 and 1000 cm−3, which is up to two orders of magnitude higher than those observed in the local universe (e.g., Masters et al. 2014; Steidel et al. 2014; Sanders et al.2016; Kaasinen et al.2017).
Far-infrared (FIR) transitions are a powerful tool for investigating the ISM. The fact that they are less affected by dust compared to optical line tracers makes them highly
advantageous for probing dusty star-forming galaxies. At wavelengths greater than 100μm, the fine-structure transitions of [CII] 157.7 μm, the [NII] 121.9, and 205.2 μm have been
used for probing ISM conditions of local and high-z galaxies (Stacey et al.1991; Wright et al.1991; Bennett et al.1994; Lord et al. 1996; Malhotra et al. 2001; Brauher et al. 2008; Nagao et al.2012; Farrah et al.2013; Zhao et al.2013,2016a,2016b; Herrera-Camus et al.2016,2018a,2018b). With an ionization
threshold of 11.3 eV, the [CII] line emission arises from the
neutral and the ionized gas. On the other hand, the two[NII]
fine-structure lines originate from fully ionized gas because the ionization potential of nitrogen (14.5 eV) is about ∼0.9 eV higher than that of hydrogen. Therefore, the ionized nitrogen [NII] lines reflect the effect of UV photons emitted by massive
young stars, with possible enhancement from X-ray photo-ionization. The combination of twofine-structure lines can be used as a tracer of electron density and this diagnostic barely depends on the electron temperature(e.g., Goldsmith et al.2015; Herrera-Camus et al.2016).
The [NII] 122 μm line emission has not been detected for
galaxies at 4<z<7 until now, which is the epoch when larger number of galaxies are beginning to form after the end of the reionization. In this Letter, we report thefirst detection of [NII]
122μm line from a quasi-stellar object (QSO)-submilimeter-bright galaxy (SMG) pair, BRI 1202−0725, at z=4.69. This compact group of BRI 1202−0725 was one of the first z>4 submillimeter-bright systems discovered(Isaak et al.1994) and
remains the archetype for major starbursts in gas-rich mergers in the early universe. It consists of an optically selected QSO, an optically faint SMG that is located 4″ (≈26 kpc) northwest of the quasar (Hu et al. 1996; Omont et al. 1996), and two
Lyα-selected galaxies in their very vicinity(Fontana et al.1996; Hu et al. 1996; Ohta et al.2000; Salomé et al.2012; Carilli et al.
2013; Carniani et al.2013). Extremely high FIR luminosities of
QSOs and SMGs (Omont et al. 1996; Yun et al. 2000; Iono et al. 2006; ~1013L
) imply vigorous star-forming activity of
≈1000Me yr−1. The system is known to have rich C-bearing emission line data sets; various rotational CO molecular lines have been detected to up J=11 (e.g., Ohta et al.1996; Omont et al. 1996; Salomé et al. 2012) in addition to bright [CII]
emissions(Iono et al.2006; Carilli et al.2013). Lu et al. (2017; hereafter,Lu17) reported the first detection of [NII] 205 μm line
emissions for both systems and measured the dust temperature (Tdust=432 K) using the line ratio between the [NII] line and CO(7–6). We add new [NII] 122 μm line detections, which
provide further constraints on the physical conditions of the ISM, namely the electron density.
We assumeH0=67.8 km s-1Mpc-1,Ω0=0.308 and ΩΛ= 0.692(Planck Collaboration et al.2015).
2. Atacama Large Millimeter/submillimeter Array (ALMA) Observations and Data Reduction
2.1. Band 6 Observations for[NII]205μm
The Band 6 observations were carried out for our ALMA Cycle 2 program. A total of 39 and 40 antennas were used with the unprojected length(Lbaseline) between 15 and 348 m (C34-2/1) on 2014 December 14 and 2015 January 4 with a total on-source time of 58 minutes.
We used four spectral windows(SPWs), each being 1.875 GHz wide. Two of them were set in the upper sideband with 3.906 MHz resolution (∼4.5 km s−1) to target [NII]205μm.
One SPW in the lower sideband was also set to 3.906 MHz resolution. The remaining SPW was set to 7.812 MHz resolution (∼9.7 km s−1), in which we detected CO(12−11) emissions both from the SMG and the QSO(M. Lee et al. 2019, in preparation). Two strong quasars J1256−0547 and J1058+0133 were chosen for bandpass calibration. J1216−1033 was the phase calibrator. Callisto was theflux calibrator.
2.2. Band 8 Observations for[NII]122μm
The Band 8 observations at 700μm were also a subset of the same ALMA Cycle 2 program. Observations used 37 or 38 antennas with the unprojected length(Lbaseline) between 21 and 783 m on 2015 June 6 through 8 and total on-source time was 112 minutes.
We used four SPWs, each being 1.875 GHz wide. Two of them were set in the upper sideband with 3.906 MHz resolution (∼2.7 km s−1) to detect [NII] 122 μm. The spectral resolution for the remaining two SPWs in the lower sideband was set to
7.812 MHz(∼5.6 km s−1). J1256−0547 was chosen as a bandpass and a phase calibrator. 3C273 and Titan were chosen for flux calibrators.
2.3. Archival Data: Band 6 Archival Data
We downloaded the archival data sets that were indepen-dently taken during ALMA Cycle 3 for [NII] 205 μm line
detection reported inLu17. The details of the observations are presented in Lu17. We calibrated the data based on the provided pipeline script. It was observed in the time-domain mode (TDM) with a spectral resolution of 15.625 MHz, corresponding to∼19 km s−1, in which the spectral sampling is a factor of≈4 coarser than our Band 6 data sets. Hereafter, we will refer the data as the“Lu data.”
2.4. Data Reduction and Analysis
We performed calibration using the Common Astronomy Software Applications package(CASA; McMullin et al.2007).
For our Band 6 and 8 data sets, we used the calibration scripts provided by the ALMA ARC members that used CASA version of 4.2.2 and 4.3.1, respectively. For the Lu data, we used CASA 4.5.2.
Images were produced byCASA task tclean. All imaging processes were handled with the 5.4.0 version. Using the natural weighting, the synthesized beam sizes are 1 43×0 84 and 0 32×0 24 for the [NII] 205 μm and [NII]122 μm
observa-tions, respectively. For the Lu data, the beam size is 0 97× 0 80.
Because different resolutions were obtained in different bands, we tried to match the resolution as much as possible. For comparison with the Lu data, we made 1 5-resolution images for both Band 6 data sets to estimate the line width and theflux. For the [NII] 122 μm data, we investigated signal-to-noise
ratios(S/Ns) over a few uvtaper parameters. We chose the uv-tapering parameter of 330 kλ with the synthesized beam of 0 44×0 38, which is the size without losing significant S/N, i.e., peak S/N from ≈7.3 (7.3) to ≈7.1 (8.2) for the SMG (QSO). We also made highly tapered [NII] 122 μm images in
order to obtain a resolution that was close to the[NII] 205 μm
data. With the uv-tapering parameter of 80 kλ, the beam size is 1 20×1 13. This gives lower peak S/Ns of ∼2–3 for both galaxies. In Section 4, we use the highly tapered images (“80 kλ-tapered map”) to evaluate potential systematic errors. For our[NII] 205 μm data, we applied Briggs weighting with a
robustness of 0.5, which gives a synthesized beam of 1 32× 0 68. We subtracted the continuum based on image datacube using imcontsub to better control the continuum shape especially for targets away from the phase center and hence to get higher S/N than using uvcontsub. We checked that the flux measured from the data after applying uvcontsub gives consistent values within errors.
We measured the flux after investigating the flux growth curves with various aperture sizes. The flux values reach the asymptotic values with aperture sizes of 1 2 and 3 0 for the [NII] 122 μm and [NII] 205 μm, respectively. Using these
aperture sizes, we derived the flux values based on Gaussian fitting using CASA task imfit.
2.5. Missing Flux
Considering the high angular resolution obtained in the[NII]
122μm observations, we explored the possibility of emission from extended regions. We investigated mock observations of a Gaussian structure component with various sizes at given configuration of C34-5 during cycle 2 using the CASA task simobserve. Figure1 shows such an experiment, when the images are created after applying the same uv-tapering parameter of 330 kλ. At an ideal condition of infinity S/N (i.e., without noise), we were able to recover more than 80% of total flux when the size of the source is extended up to a size of ≈8 kpc.
We estimated the sizes of [NII] emitting regions using the
CASA task imfit. We used the natural-weight maps of the [NII] 122 μm. We could only constrain the size of the QSO,
which is 0. 43( 0. 15)´ 0. 23( 0. 18) and the upper limit for the SMG, which is 0 38×0 23. For comparison, the beam-deconvolved [NII] 205 μm sizes are 0 59(±0 19)×0 42
(±0 21) and 0 78(±0 18)×0 62(±0 38) for the QSO and the SMG, respectively, from the Briggs-weighted maps. While there is a hint of smaller sizes for the [NII] 122 emissions
compared to the[NII] 205 from these measurements, we note
that the uncertainties are also large. At least from the Gaussian fitting, we conclude that both [NII] lines are emitted from
regions of similar sizes comparable to or smaller than the[CII]
emitting regions, which are≈2–3 kpc in scale radius (Carniani et al. 2013). Lu17 reported extended emissions (i.e., ∼9 kpc (≈1 4) for the QSO and 14 kpc (≈2 1) for the SMG) in [NII]
205μm line, which are larger than the estimates from our data. We could only constrain the [NII] 205 μm size for the QSO
from the Lu data, 0 81(±0 21)×0 49(±0 34), which is consistent with our data(the size before deconvolution is 1 21 (±0 10)×1.00(±0 07)). For the SMG, the size before beam deconvolution is 1 58(±0 19)×0 80(±0 06), but the fit gives only an upper limit of the size to be 1 50×0 28. It may be worth noting that our data is deeper in terms of the point source sensitivity by 1.4×. Considering this, it is less likely that a significant amount of emission is coming from extended
regions (>10 kpc). Therefore, we rely on the flux measure-ments without any correction.
3. Results
For the [NII] 205 line emission, we found that our
measurements are consistent with the Lu data within the uncertainties, in terms of the peak positions, the line widths, and the luminosities. The spectra of all these data sets are shown in Figure 2. Pavesi et al.(2016) used our Band 6 data
and reported theflux measurement briefly, which we reconfirm with the same data set but with different imaging processes. We note thatLu17reported different flux values i.e., 0.99±0.02 and 1.01±0.02 for the SMG and the QSO, respectively, in which they used different aperture sizes for individual galaxies as opposed to ours. The flux values with the same flux extraction methods to ours using the Lu data are 1.07±0.16 (SMG) and 0.81±0.10 (QSO). These are consistent with our measurements listed in Table1within the errors. However, all flux values using the TDM data tend to be smaller (larger) for the SMG (QSO) compared to our data (in frequency-domain mode). While it is difficult to investigate the origin of the difference, we emphasize that we measured the linefluxes after careful analysis of theflux growth curves from Gaussian fitting and aperture photometry.
From our Band 8 observations, the [NII]122μm line is
detected in both of the SMG and the QSO. The spectra for individual galaxies are shown in Figure 2. The line intensity maps are shown in Figure3with the peak positions of the[CII]
line, which are consistent with each other, and the peak positions of[NII] and [CII] emissions are consistent with each
other considering the S/Ns and the beam sizes.
As described in Table1, the width of the[NII] 122 μm line
for the QSO is 613±133 km s−1, which is broader roughly by a factor of two than those observed in the[NII] 205 μm line
(297 ± 104 km s−1), [CII] (300 ± 28 km s−1), and CO lines (≈300–350 km s−1) in the literature (e.g., Salomé et al.2012; Carniani et al. 2013). We preformed the following tests to
verify whether the different line width is an effect of the different analyses. First, we did notfind systematic differences in the line profile between the tapered and the natural-weight map. Second, we found that the line profile is robust regardless of continuum-subtraction methods: the continuum subtraction based on the 1D spectrum using the 0 6-aperture is consistent with the imcontsub and uvcontsub. Therefore, we conclude that the different line widths between[NII] 122 and
[NII] 205 for the QSO are likely real. This may indicate higher
electron densities at higher velocities for the QSO, which we will discuss in the following section.
4. Discussion
We estimate the electron density using the observed ratios between twofine-structure lines of N+. We used the PYNEB package(Luridiana et al.2015) to perform the calculations. The
observed 122μm/205 μm line luminosity ratios are 1.44± 0.36 and 3.89±0.71 for the SMG and the QSO, respectively. These correspond to the electron densities of26-+1112(SMG) and
-+
134 39
50cm−3(QSO) at the electron temperature of Te=8000 K
(Figure4), which is used in local spiral galaxy studies
(Herrera-Camus et al.2016).
We evaluate potential systematic errors coming from extract-ing the flux values in the following manner. First, using the
Figure 1.Expected missingflux assuming a Gaussian distribution at given flux using CASA simulation with uv-tapering of 330 kλ for an ideal case without noise.
80 kλ-tapering map, the [NII] 122 μm fluxes are 1.19±0.49
and 1.93±0.58, for the SMG and the QSO, respectively, using the same aperture size of 3 0. This is consistent with the value obtained from the 330 kλ-tapered maps measurement within errors. Second, we also measured the linefluxes using a smaller aperture size of 1 8 for the [NII] 205 μm data, which is
determined after taking into account the emitting size of the
[NII] 122 μm line at most (≈1 2 from the growth curve) and the
[NII] 205 μm beam size. The flux values from the aperture
photometry are 1.00±0.07 and 0.57±0.04 and Jy km s−1for the SMG and the QSO, respectively, providing ne=41-+1517
(SMG) and199-+6388cm−3(QSO). While these estimates give the
lower limit of the electron densities when the size difference between two[NII] lines are large (i.e., the [NII] 122 μm emitting Figure 2.Detection of[NII] 122 (top row) and [NII] 205 (bottom row) lines from SMG (left panels) and QSO (right panels) in blue solid lines. Top row: the [NII]
122 spectra. The[NII] 122 μm spectrum extracted at the peak position in 100 km s−1resolution from the 330 kλ-tapered (i.e., 0 44 × 0 38) cubes. We overplot [CII]158 μm line from Carniani et al. (2013) with the base level shifted to 1.5 for clarity. Bottom row: the [NII] 205 spectra in 100 km s−1resolution. The spectra are obtained from the 1 5×1 5 resolution cubes at the peak positions. Overlaid orange dashed lines show the [NII] 205 μm detection from another independent data set
reported inLu17, which we re-analyzed for our comparison. We matched the resolution to 1 5×1 5 resolution for this data. We overplot [CII] 158 μm line from
Carniani et al.(2013) with the base level shifted to 1.0 for clarity. The velocity centers of the spectra are based on the redshifts from [CII] 158 μm observations in
Carilli et al.(2013).
regions being much smaller than the[NII] 205 μm emission), they
serve as a gauge for the central regions. Third, if we perform 2D-Gaussianfit for the the [NII] 122 μm emission using 3″-aperture
for the 330 kλ map, the flux values are 2.18±1.01 and 1.68± 0.77 for the SMG and the QSO, where the uncertainties then become quite large. From these potential systematic errors, we conclude that the derived electron densities can increase up to by a factor of ∼1.5 for the QSO and ∼3 for the SMG.
Variation in electron densities in different galaxies have been argued in several studies. For example, Herrera-Camus et al. (2016) argued that the electron density correlates with the star
formation surface rate density for local spirals using the same [NII] tracer, based on the spatially resolved emission at ∼kpc
scale. Kaasinen et al.(2017), using the optical tracer of [OII]
for z∼1.5 star-forming galaxies, discussed that the star formation rate (SFR) is the main driver of varying electron densities. On the other hand, Sanders et al.(2016) did not find a
clear trend of electron density with SFR. We estimated the rest-frame 123 μm dust continuum (Band 8) sizes based on the uvmultifit (Martí-Vidal et al. 2014), which are ≈1 kpc for
both galaxies. Considering similar SFRs of ≈1000Meyr−1 (e.g., Salomé et al.2012) and similar dust sizes, we do not find Table 1 Flux Measurements Target SMG QSO Fline a FWHM Lline Fline a FWHM Lline (Jy km s−1) (km s−1) (×109L e) (Jy km s−1) (km s−1) (×109Le) (1) (2) (3) (4) (5) (6) (7) [NII]122μm 1.13±0.27 871±228 2.71±0.65 1.62±0.27 613±133 3.89±0.65 [NII]205μm 1.32±0.11 1009±147 1.86±0.15 0.70±0.05 297±104 0.98±0.07 Note. a
We measured theflux with an aperture size of 1 2 and 3 0 for [NII] 122 μm and [NII] 205 μm, respectively.
0
Figure 3.Top row: the line intensity of[NII] 122 μm for SMG (top-left panel) and QSO (top-right panel). Contour lines are starting from 2σ, in steps of 2σ where
1σis 0.13 and 0.10 in Jy km s−1for the SMG and the QSO, respectively. We also added negative contours of−4σ and −2σ in gray dashed lines. The beam sizes after uv-tapering are shown in whitefilled ellipses, which is 0 44×0 38. Bottom row: the line intensity of [NII] 205 μm for SMG left panel) and QSO
(bottom-right panel). Contour lines start from 2σ, in steps of 2σ where 1σ is 0.04 and 0.03 in Jy km s −1for the SMG and the QSO, respectively. The beam size is 1 32×0 68. All panel sizes are 5″-width. The cross markers are the peak positions of the [CII] 158 line and the ellipse filled in black is the beam size of the [CII]
the dependence of electron density on the SFR or SFR surface density on a global scale.
The difference may be indicative of different phases for the black hole growth and/or different gas distributions in the SMG and the QSO. One possible scenario is that gas may be more centrally concentrated for the QSO compared to the SMG. This is counter-intuitive from the preferred formation scenario of elliptical galaxies and the connection between SMGs and QSOs (e.g., Hopkins et al.2008; Toft2014) where (SMG-like) heavily
dust-obscured compact phase with“denser” ISM precedes the optically bright QSO phase. However, so far no conclusive argument has been made for the connection. The different[NII] line widths in
the QSO might indicate higher line ratios of[NII] 122/205 in the
high-velocity components, thus higher electron densities. If the above scenario is considered, this may be ascribed to gas in the core perhaps at the inner peak of the rotation curve, possibly close to the black hole. We investigated whether the line profiles are different in the center(r < 0 2) and outer region (0 2 < r < 0 4). But we could not find any statistically significant difference in thefitted line widths, partially owing to the low S/N.
Alternatively, the high-density gas in the QSO may be a signature of (moderately dense) ionized outflowing gas. We note that there is a“red wing” in the [CII] line profile (Carilli
et al.2013; Carniani et al.2013), which may be associated with
a faint companion or with an outflow. Observations of active galactic nuclei-driven galactic outflows in the local universe (e.g., Sakamoto et al.2009; Kawaguchi et al.2018) support the
idea of denser gas in the outflowing wind, perhaps due to gas compression. As it is difficult to obtain the matched resolution spectra for both galaxies owing to the sensitivity limit, future deeper high-resolution observations are needed to confirm.
For more comparison, we compiled the available data sets for various types of galaxies including our Galaxy(MW) and local galaxies as shown in Figure4. We note that these local measurements are in most cases based on spatially resolved emissions and they have a range of electron densities within the
galaxies, while our case is for the global average value of the system, assuming that both [NII] lines are coming from the
same region. The SMG has comparable electron density compared to those observed in the Galactic Plane(Goldsmith et al. 2015) and the average values of nearby, star-forming
galaxies (Herrera-Camus et al. 2016) using the same tracers,
even though the SFRs differ on two to three orders of magnitude. Meanwhile, the QSO shows a similar value to the starburst galaxy like M82(Petuchowski et al.1994) and NGC
1097 (Beirão et al. 2012). It is also similar to the typical ne
values found in the central regions of nearby galaxies (Herrera-Camus et al. 2016), which are represented by the last two
bins in the electron density distribution in the right panel of Figure4.
There are limited number of higher redshift(z > 1) galaxies with the[NII] line detections (e.g., Zhang et al. 2018; Novak et al. 2019). In Zhang et al. (2018), they estimated the lower
limits of electron densities for lensed, dusty starbursts at a range of z=1–3.6 based on stacking analysis, which is ne>100 cm−3. Given the range of electron densities in the BR1202−0725 system, the stacking analysis may have missed a portion of dusty star-forming galaxies with low electron densities like BR1202−0725 SMG. Novak et al. (2019) also
reported higher lower limit of electron density(ne> 180 cm−3) for a QSO at z=7.5. Similarly, but using the rest-frame optical lines of[OII] and [SII], several studies reported higher electron
densities on average compared to local galaxies ranging between ∼100 and 250 cm−3 (e.g., Sanders et al. 2016; Kaasinen et al. 2017), but there are scatters in the
measure-ments. It may be worth noting that these optical lines can trace slightly denser gas in the 100<ne/cm−3<104 range compared to the [NII] lines10<ne cm-3<500. Thus, it is likely that rest-frame optical lines nemeasurements(e.g., [OII] and [SII] lines) yield, on average, higher electron density
measurements.
Figure 4.Left panel: the[NII] line luminosity ratio as a function of electron density. The two solid curves are for different electron temperatures (Te=8000 and
25,000 K). The line ratios for the SMG and the QSO are shown as green solid and purple dashed lines, respectively. We also plot the observed line ratio for local populations. The line ratios obtained from local spirals(Herrera-Camus et al.2016) are also plotted. Right panel: the histogram for the distribution of electron density,
based on the observed line ratio for Herrera-Camus et al.(2016) for comparison with the BR1202−0725 system. The remaining data sets are retrieved from Bennett
et al.(1994; Milky Way(MW): COBE), Goldsmith et al. (2015; MW: galactic plane), Parkin et al. (2013; M51), Petuchowski et al. (1994; M82), Parkin et al. (2014; Cen A), Hughes et al. (2015; NGC 891), Beirão et al. (2012; NGC1097), Xiao et al. (2018; NGC3665, early-type galaxy), and Díaz-Santos et al. (2017; local luminous infrared galaxies).
In this respect, there may be extremely high electron densities in central regions or elsewhere perhaps with extreme SFR densities where the [NII] lines are not viable for
measuring electron densities. As seen from the resolved measurement in the local galaxies (e.g., Goldsmith et al.
2015; Herrera-Camus et al. 2016), we do not expect these
galaxies to have uniform electron gas densities across the galaxy, but rather to follow some sort of distribution (e.g., a log-normal distribution like the diffuse warm ionized medium). If such is the case, the[NII]122/205 line ratio can only probe a
part of the whole density distribution. Therefore, it could be that both the QSO and SMG have similar high mean electron densities but different distribution widths, which could be the reason why the [NII]-based ne measurement in the SMG are lower than that in the QSO. To confirm the existence of extremely high-density regime, we need other lines instead to trace such regime with higher critical density, such as[NIII] or
the combination of[OIII]52 μm and [OIII]88 μm, which can be
only accessible from space telescopes.
Considering the existence of the heavily obscured galaxy such as Arp 220 and the fact that shorter wavelengths tend to be more affected by the dust, the reduction of the intrinsic value of the[NII]122/205 line ratio of the SMG compared to the QSO
might be(at least partially) owing to the extremely dusty nature of the SMG. Deeper high angular resolution observations at various wavelength would confirm the true nature of the SMG and the QSO.
The success of the [NII] 122 and 205 μm detections at
z=4.69 demonstrate the power of future systematic surveys of extreme starbursts at z>4, using these lines for probing the ISM conditions, and the effects on surrounding environments in terms of electron densities.
We deeply appreciate the anonymous referees for the fruitful discussions and suggestions for the revision of the Letter. We thank Rodrigo Herrera-Camus for providing the data set of local galaxy survey for[NII] lines and fruitful discussions. We
also thank Zhi-yu Zhang for the helpful discussions on the treatment of theflux measurement. This Letter makes use of the following ALMA data: ADS/JAO.ALMA #2013.1.00259.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. S.C. is supported from the ERC Advanced Grant INTERSTELLAR H2020/740120. This work was supported by NAOJ ALMA Scientific Research grant Nos. 2018-09B. R.M. acknowledges ERC Advanced Grant 695671“QUENCH.”
Facility: ALMA.
Software: astropy(Astropy Collaboration et al.2013).
ORCID iDs
Minju M. Lee https://orcid.org/0000-0002-2419-3068
Tohru Nagao https://orcid.org/0000-0002-7402-5441
Carlos De Breuck https://orcid.org/0000-0002-6637-3315
Stefano Carniani https://orcid.org/0000-0002-6719-380X
Giovanni Cresci https://orcid.org/0000-0002-5281-1417
Bunyo Hatsukade https://orcid.org/0000-0001-6469-8725
Ryohei Kawabe https://orcid.org/0000-0002-8049-7525
Kotaro Kohno https://orcid.org/0000-0002-4052-2394
Filippo Mannucci https://orcid.org/0000-0002-4803-2381
Alessandro Marconi https: //orcid.org/0000-0002-9889-4238
Kouichiro Nakanishi https: //orcid.org/0000-0002-6939-0372
Toshiki Saito https://orcid.org/0000-0002-2501-9328
Yoichi Tamura https://orcid.org/0000-0003-4807-8117
Hideki Umehata https://orcid.org/0000-0003-1937-0573
Min Yun https://orcid.org/0000-0001-7095-7543
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